Usally played with 7-10 people in a circle. There are many game types. Here are a few:
When the people stand in a circle, and the sack is kicked, if you miss the sack (other than on a serve) you are out. If the sack hits the middle or "no mans land" nobody is out. When there are 2 people left, the first person to get eliminated holds the sack, both players touch it, and the sack is thrown up. After throwing up, whoever misses the sack first/hits a bad hit is out.
When a group of friends kick to each other, and play sack. No rules, just kicking.
Everyone in the circle kicks the sack once, then passes it to someone else. If the sack hits the ground, or the same person twice before everyone has kicked it, you lose. Every 1 sack is when everyone in the circle hits the hackey sack once.
No rules - friends kick to eachother showing off their best moves, i.e. Jester, otr, high top, russian jesters.. etc etc.
(2)FreeStyle - Pass the hackysack here kris, oh wow you failed.
(3)Sacked - Damn, we got 7 sacks there before mario miss kicked the hackeysack.
(4)ShowOff - HOLY SHIT, he just tripple russian jestered into that hackysack into an otr!
Celebrate Hacky Sack Day
There are also a variety of other gmaes to play with a hacky sack such as
Killer Hack- a certain amount of people must touch the hack and then anyone can grab it out of the air and whip it at another person in the circle. If you are hit you are out for that game.
Power Hack- Everyone tried to hit the hacky sack as high into the air as possible, and make amazing saves to keep it going.
4 Square- Normally played with hands and a large ball that bounces, the rules are very similar but you play with a hacky sack and your feet. If you let the Hacky sack land in your square you are out and someone new comes in.
Hacky sack is a sociable game and can be thought of as a minor sport, no doubt that this game will eventually make it to the olypics as many irregular sports/games have. I dont know if there will be many competitors though because many hackyers are stoners, and you are not allowed drugs during the olympics.